strong vibration and knocking sound

[ahatch3]

hi, we need some help here with our 89 camaro, 86 block, 6 cyl., automatic. it has a strong vibration, i'm talking engine, exhaust, and body vibration. we've just changed the harmonic balancer. this did need replacing even tho it did nothing on taking care of the problem. also have changed the cat and tightened the exhaust headers down, as they were all loose.
now after all this the car has acquired a knocking sound like a rod bearing. the car still has some power, not like it use to. and the vibrating seems to like build up till it shifts each time.
if anyone has any kind of ideas we may have over looked, let us know. thanks so much.

[89IROC&RS]

perhaps a dead cylender??? does it shake when the car is stationary and running? that would mean its an engine vibration, which would be a misfire, possibly a ignition related issue.